1. Field of the Invention
The present invention relates to incorporation of nonlinear optical chromophores into polymer mattrices and, more particularly, pertains to polymers containing polyene-bridged second-order nonlinear optical chromophores and devices incorporating the same.
2. Description of the Related Art
Organic second-order nonlinear optical (NLO) materials have received increasing attention for applications involving signal processing and communications. Second-order NLO chromophores (also referred to as chromophore hereafter) can provide higher electro-optic activity than inorganic materials such as lithium niobate. (xe2x80x9cElectro-optic polymer modulators for 1.55 xcexcm wavelength using phenyltetraene bridged chromophore in polycarbonatexe2x80x9d, M.-C. Oh, et al., Applied Physics Letters, vol. 76, no. 24, pp 3525-3527.) However, it remains a challenge to incorporate high xcexcxcex2 (xcexc-dipole moment, xcex2-first hyperpolarizability) NLO chromophores into thermally stable and low optical loss polymers.
There are two basic ways of incorporating chromophores into polymers. One is the guest-host approach where chromophore and host polymer are physically mixed and dissolved in a solvent to make a solution for film spin-casting. The other approach is to chemically (covalently) attach chromophore to polymer chains (hereafter refered as covalant approach). The guest-host approach has been used in demonstration/first generation electro-optic devices (xe2x80x9cElectro-optic polymer modulators for 1.55 xcexcm wavelength using phenyltetraene bridged chromophore in polycarbonatexe2x80x9d, M.-C. Oh, et al., Applied Physics Letters, vol. 76, no. 24, pp 3525-3527) and a dynamic thermal stability of 120xc2x0 C. was obtained for the CLD-1/amorphous polycarbonate material. However, this stability is not high enough for long term operation (e.g., room temperature for 5 years or 85xc2x0 C. for 1,000 hours). Studies on various guest-host systems have revealed that there are several intrinsic problems with the guest-host approach. First, high glass transition temperature (Tg) host polymers are usually very insoluble. Second, compatibility between NLO chromophores and high Tg hosts is generally poor, which leads to phase separation, low poling efficiency and high optical loss. Third, thermal stability obtainable in guest-host systems is very limited because chromophores are not locked to polymer chains by covalant bonds.
High xcexcxcex2 chromophores such as the FTC and CLD chromophores disclosed in the above-referenced patent applications are highly polar molecules. Strong electrostatic interaction happens when these chromophores are close to each other. Such interactions strongly attenuate the efficient induction of acentric chromophore order (hence, electro-optic activity) by electric field poling. When chromophore molecules are bonded to polymer chain(s), they can no longer move freely to form a separate phase or tightly packed aggregates.
According to the present invention, high thermal stability of CLD/FTC materials is obtained by using rigid monomers and high crosslinking density, and by locking two ends of chromophore to polymer chains. This high thermal stability has been observed during. extensive studies on a covalent approach to incorporating FTC and CLD types of chromphores into polyurethane 3-dimensional networks.
Optical loss is another important issue that must be considered in NLO polymer design and synthesis. Polyurethanes are intrinsically lossy at the two important communication wavelengths 1.3 xcexcm and 1.55 xcexcm due to the existence of Nxe2x80x94H group. According to the present invention, in order to reduce optical losses at 1.3 xcexcm and 1.55 xcexcm, other crosslinking agent(s) that do not introduce significant absorption at the two wavelengths are used.
According to the present invention, new classes of polymers are provided which address the thermal stability and optical loss issues in electro-optical materials. The chromophores of the present invention are functionalized and incorporated into a series of polymers ranging from linear polyesters, linear poly[ester-imide]s, chain-crosslinked polyesters/poly[ester-imide]s, to crosslinked star-shaped/dendritic macromolecules. The linear polymer approach offers more straightforward synthetic scheme and moderate thermal stability, while the crosslinked approachs provide higher thermal stability due to double-end locking of chromophores and cross-linking of polymer segments. Also, according to the present invention, star-shaped/dendritic macromolecules are provided which reduce interchromophore interaction between chromophores, and hence afford low optical loss and higher order of poling-induced chromophore dipole alignment in addition to high thermal stability.
An exemplary preferred polymer according to the present invention is a linear chain polymer formed from a dihydroxy-functionalized polyene-bridged chromophore, a diacid or diacid dichloride, and a diol. In a preferred embodiment, the diol structure also includes at least one imide group.
Another exemplary preferred polymer according to the present invention is a crosslinked polymer comprised of a linear chain polymer formed from a dihydroxy-functionalized polyene-bridged chromophore, a diacid or diacid dichloride and a diol. The crosslinking is realized by thermally induced cyclolization of trifluorovinylethers which are attached to the diol co-monomer and/or chromophore in the linear chain polymer chain. In a preferred embodiment, the diol structure also includes at least one imide group.
Another exemplary preferred polymer according to the present invention is a crosslinked polymer made from crosslinkable star-shaped high molecular weight (larger than 2,000) structure containing polyene-bridged chromophores. In a preferred embodiment, the crosslinking groups are multiple trifluorovinylether groups located at the periphery of the star-shaped structure.
Another exemplary preferred polymer according to the present invention is a crosslinked polymer made from crosslinkable dendritic structure having a chromophore core and one or more dendrons. In a preferred embodiment, the crosslinking groups are multiple trifluorovinylether groups located at the periphery of the dendritic structure.
According to a preferred embodiment of the present photodegradation is eliminated by removing oxygen from the material in the device and from the environment of the device. An exemplary preferred technique for providing this protection according to the present invention is to hermetically package the device in a container filled with an inert gas. Another exemplary preferred technique for providing this protection according to the present invention is to insulate the device from air (oxygen, in particular) by coating the electro-optic device with a polymeric material which has a very low permeativity for oxygen.
The NLO materials of the present invention are suitable for a wide range of devices. Functions performed by these devices include, but are not limited to, the following: electrical to optical signal transduction; radio wave to millimeter wave electromagnetic radiation (signal) detection; radio wave to millimeter wave signal generation (broadcasting); optical and millimeter wave beam steering; and signal processing such as analog to digital conversion, ultrafast switching of signals at nodes of optical networks, and highly precise phase control of optical and millimeter wave signals. These materials are suitable for arrays which can be used for optical controlled phased array radars and large steerable antenna systems as well as for electro-optical oscillators which can be used at high frequencies with high spectral purity.